Household ammonia is alkaline, like lye but less so, and less dangerous. I think it was Jobst Brandt who used to recommend it. The difficulty is getting it up into the stem-steerer tube interface where the galvanic corrosion is. Never had to try it but since it hasn't been mentioned yet, I thought I would.

Then there is CLR. One of the FAQs was if it could be used to remove rust from a car. The company's answer is, "We do not recommended using CLR on a car. The acids in our product should not be used on aluminum or any painted/coated surface. You can safely remove rust from car wheels (if rims are made of chrome or stainless steel only, no alloys)." That it will damage aluminum is, of course, a good thing in that you are trying to eat away at the stem to release it. That it can damage paint is not a good thing.